1. Field of the Invention
The present invention relates to a magnetic bearing device for use with a motor-combined structure, and more particularly to a magnetic bearing device having magnetic bearings which supports a rotatable shaft that is rotated by a motor armature mounted on the rotatable shaft and a motor stator disposed around the motor armature.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows a conventional magnetic bearing device for use with a motor-combined structure. As shown in FIG. 1, the motor-combined structure has a rotatable shaft 11 that is rotated by a revolving magnetic field generated by a motor stator 13 which is disposed around a motor armature 12 mounted centrally on the rotatable shaft 11. The motor armature 12 and the motor stator 13 make up a motor. The magnetic bearing device has magnetic bearings 14A, 14B disposed around the rotatable shaft 11 one on each side of the motor stator 13. Each of the magnetic bearings 14A, 14B comprises a magnetic member 15 fixed to the rotatable shaft 11 and electromagnets 16 for producing magnetic fluxes to apply magnetic attractive or repulsive forces to the magnetic member 15 to levitate and hold the rotatable shaft 11 in a predetermined position.
Displacement sensors 17A, 17B for detecting a positional displacement of the rotatable shaft 11 are disposed respectively near the magnetic bearings 14A, 14B. The displacement sensors 17A, 17B supply detected positional displacement signals to a magnetic bearing controller 18. In the magnetic bearing controller 18, a displacement sensor signal detector 18A detects the detected positional displacement signals from the displacement sensors 17A, 17B, and a compensating circuit 18B generates a signal for levitating and supporting the rotatable shaft 11 in a desired target position from an output signal from the displacement sensor signal detector 18A. The signal generated by the compensating circuit 18B is amplified by a driver 18C, which supplies the amplified signal to the electromagnets 16. The rotatable shaft 11 is levitated and supported in the desired target position based on positional displacements of the rotatable shaft 11 that are detected by the sensors 17A, 17B.
The magnetic bearing controller 18 is supplied with electric power directly from a commercial three-phase AC power supply 20, and the motor stator 13 is supplied with electric power from the AC power supply 20 via a switch 21.
The magnetic bearing controller 18 converts the supplied electric power to electric power for generating control forces to act on the rotatable shaft 11. The converted electric power is supplied from the magnetic bearing controller 18 to the electromagnets 16.
When the conventional magnetic bearing device starts to operate, the magnetic bearing controller 18 is activated to control the magnetic bearings 14A, 14B to levitate the rotatable shaft 11. Thereafter, the switch 21 is closed to supply electric power from the AC power supply 20 to the motor stator 13. The motor armature 12 fixedly mounted on the rotatable shaft 11 starts rotating under a revolving magnetic field generated by the motor stator 13. A certain period of time after the motor armature 12 starts rotating, the rotatable shaft 11 rotates at a rated rotational speed.
If the voltage of the electric power supplied to the AC power 20 varies for some reason while the motor is in operation, then the motor is subject to a change in the voltage of the electric power supplied thereto which is commensurate with the variation of the voltage of the electric power supplied to the AC power supply 20. Specifically, because the motor stator 13 is connected to the AC power supply 20 via the switch 21, the motor directly undergoes the variation of the voltage of the electric power supplied to the AC power supply 20. When the voltage applied to the motor stator 13 varies, the drive torque applied to rotate the motor armature 12 also varies. The variation of the drive torque causes a variation of unbalanced magnetic forces produced between the motor armature 12 and the magnetic poles of the motor stator 13. The variation of unbalanced magnetic forces is transmitted through the rotatable shaft 11 to the magnetic bearings 14A, 14B.
In view of possible variations of unbalanced magnetic forces produced between the motor armature 12 and the motor stator 13 due to changes in the power supply voltage applied to the motor, the magnetic bearing controller 18 is arranged to change its output characteristics in order to prevent such variations from adversely affecting the levitation control performed thereby. Specifically, if the voltage from the AC power supply 20 changes, then the magnetic bearing controller 18 adjusts its output characteristics to provide control characteristics depending on the power supply voltage or changes control characteristic settings based on the supplied voltage.
Inasmuch as the motor armature of the motor is integrally fixed to the rotatable shaft, the motor operates normally when the motor armature rotates at the rated rotational speed, and the magnetic levitation control characteristics of the magnetic bearing device are determined on the assumption that unbalanced magnetic forces produced between the motor armature and the motor stator are substantially constant when the motor armature rotates at the rated rotational speed. If the unbalanced magnetic forces produced between the motor armature and the motor stator are changed, then the magnetic levitation control characteristics of the magnetic bearing device vary, possibly resulting in a control failure in the worst case.
If a power supply voltage that is available to the installation site for the magnetic bearing device cannot be specified, or if it is assumed that the voltage applied to the magnetic bearing device is variable over a certain wide range for the purpose of using the magnetic bearing device in numerous applications, then control characteristics of the magnetic bearing device tend to vary as a result of a variation of the unbalanced magnetic forces produced in the motor. However, it is tedious and time-consuming to adjust or modify the magnetic bearing device each time when control characteristics of the magnetic bearings vary.
It is therefore an object of the present invention to provide a magnetic bearing device for stably controlling magnetic bearings at all times without having to readjust control characteristics of magnetic bearings and change settings thereof.
To achieve the above object, there is provided in accordance with the present invention a magnetic bearing device for use with a motor-combined structure, comprising magnetic bearings for supporting a rotatable shaft out of contact therewith, a motor for rotating the rotatable shaft, a common power supply for supplying electric power to the magnetic bearings and the motor, and a motor drive power supply for limiting variations of the electric power supplied to the motor to at most a predetermined level with respect to variations of electric power supplied to the common power supply.
The motor drive power supply controls the voltage of electric power to be supplied to the motor. Even if the voltage of electric power supplied to the common power supply varies over a certain range, the motor drive power supply can hold variations of the voltage of the electric power to be supplied to the motor within a predetermined range. Regardless of variations of the voltage of the electric power supplied to the common power supply, therefore, the motor is supplied with electric power at a substantially constant voltage. Consequently, unbalanced magnetic forces produced between a motor armature and a motor stator of the motor can be held at a constant level or less at all times. The magnetic bearings which levitate and support the rotatable shaft can be controlled stably regardless of variations ofthe voltage ofthe electric power supplied to the common power supply. The unbalanced magnetic forces held at a constant level or less are prevented from adversely affecting the magnetic bearings via the rotatable shaft, making it unnecessary to readjust or replace the magnetic bearings. The magnetic bearings can thus be operated stably even in installation sites where the voltage of the common power supply varies in a relatively wide range.
Preferably, the motor drive power supply comprises an input/output voltage monitor unit for detecting an input voltage from the common power supply and an output voltage to be applied to the motor, a processing unit for generating a control signal to make the output voltage constant based on a variation of the input voltage supplied from the common power supply to the motor drive power supply, and an output unit for supplying a constant output voltage to the motor regardless of variations of the input voltage from the common power supply.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.